The present invention relates to a method and a device for the preparation of mould sand using a mixing process in a mixer, wherein the preparation takes place at least partially under vacuum.
The preparation of sand for the manufacture of moulds is intended to produce the correct mixing ratio of grain sizes and of proportions of quartz sand, binding agent, coal dust, optionally other additives as well as recycled and new sand, to homogenise the mixture and, in this context, to coat the grains with the binding agent to a large extent, to adjust the correct moisture content, to adjust the correct temperature of the mould sand and finally to convey the ready-prepared sand to the places of use.
Recycled sand generally has an increased temperature, for example, of between 100° C. and 140° C. Because sand temperatures above approximately 50° C. can present considerable problems to the moulding equipment, and at excessively high temperatures, uncontrollable evaporation losses over the distance between the mixer and the moulding equipment can lead to fluctuations in moisture in the ready sand, the sand must be cooled in this case. Conveyor belt coolers, through which the sand passes continuously as a result of vibrating or agitating movements of a screen mesh, are mostly used for this purpose.
An alternative cooling method has been proposed in DE 295 24 03 C2. This cooling method provides the concurrent preparation and cooling of clay-bound foundry moulding sands in a vacuum mixer. In this context, the individual components are initially placed into the mixer. After a short prehomogenizing process, the temperature and moisture of the mixture are measured and the necessary quantity of water is added. Finally, during the preparation process, the pressure in the mixture is gradually reduced. As soon as the pressure corresponds to the vapour pressure curve of water, the water in the sand begins to boil and removes the necessary evaporation heat from the sand. As a result, an extremely effective cooling method is achieved in an economical manner. According to the disclosure of DE 199 45 569, alongside the cooling effect on the mould sand, the described cooling of mould sand under vacuum also leads to an increase in quality of the mould sand prepared. Accordingly, DE 199 45 569 suggests preparation under vacuum even for recycled sand which has already been cooled.
It has been shown that the best mould sand quality can be achieved with the assistance of vacuum preparation. However, the known stages of the method and the devices and/or peripheral equipment used and their method of operation are only suitable to a limited extent or not at all for use in a fully automated foundry moulding plant. According to experience, error-free and above all economically optimized operation is not possible with the known method.
This is because, among other factors, the filling and emptying of the mixer is very time-consuming. For the ventilation of the mixer, a mixer cover is provided, in all known versions, which, in the closed condition, must be vacuum-tight to allow vacuum operation, and which is opened for the purpose of charging the mixer. In this context, the cover is generally connected to the mixer in a rotatable manner by means of a rotating axis. The cover can be designed in such a manner that it is rotated outwards or inwards in order to open the container. If it is rotated inwards, the closing mechanism must press the cover outwards against the sealing surface of the mixer with considerable force during vacuum operation. In order to manufacture the closing mechanism in an economical manner, the mixer cover must therefore be very small, because only a small force need then be applied by the closing mechanism.
If the cover is opened outwards, the closing mechanism can be designed to be weaker and can therefore be manufactured more economical ly, because the necessary pressing force can be generated by the pressure difference between the mixing container and the surroundings alone. However, with this version, the design must take into consideration that sufficient rotational clearance remains above the cover so that the cover can be opened without it coming into contact with any objects. Accordingly, dosage funnels or dosage devices must be attached at an appropriate distance above the mixer opening. This distance necessarily increases in proportion with the size of cover. However, when filling the mixer, it is important to ensure that the sealing surface of the charging opening remains as a free from contamination as possible, in order to guarantee a vacuum-tight closability. However, the probability of contamination of the sealing surface increases considerably as the distance between the dosage funnel and filling opening and/or the dropping height of the substance to be charged increases. For this reason, it is currently assumed, that a generic mixer cannot be manufactured economically with a large charging opening. Accordingly, the known mixers all possess only a relatively small opening in the pressure casing of the mixer, and, in the case of the equipment known so far, the mixture is added only in a very fine flow. This results in a very long charging time and therefore also a very low plant performance. If the mixer is loaded too quickly, an excess air pressure rapidly builds up as a result of compression of the air in the mixer. This excess pressure generally leads to the emission from the mixer of dust-like components of the mixture, which can, for example, also be deposited on very sensitive machine components, such as cogwheels and gaskets. This means that the plant must be cleaned more frequently which, once again, is associated with higher costs and undesired interruptions of operation. For this reason, it has generally been considered that, on the one hand, the charging rate cannot be further increased, because larger input openings cannot be realized in an economical manner. And, on the other hand, it has been considered that a larger charging rate will lead to the disadvantages described and must therefore be avoided.